There are many devices and much background material pertaining to hearing aids that increase the amplitude of the sound impinging on the eardrum. Some of these operate by bone conduction. These are external amplifying devices which transmit sound through the bones behind the ears. As external devices, they can be unattractive, and can interfere with eyeglasses. Functionally, such devices have the disadvantage of not providing more normal hearing and taking advantage of what adequate hearing a user has.
Most hearing aids have their amplifying transducer fitted into the auditory meatus (the external auditory canal). This blocks the natural amplitude gain at the eardrum (tympanic membrane) produced by the auditory canal and causes substantial or complete amplitude loss so the user must rely totally on electronic amplification.
One problem with such devices is that its is hard to provide frequency band matched amplification electronically to match the patient's hearing where he still has reasonably normal hearing at least at lower frequencies of the natural sound field around him without feedback distortion and over amplification of the background noise. For good or acceptable voice word comprehension, many patients need only some boost at higher frequencies, which is where most of the hearing loss in later life occurs.
In particular, the hearing loss at higher frequencies is often in the range of 30 db or more. Amplification at such levels often results in whistle and feedback. This is generally dealt with by sealing the hearing aid to the wall of the auditory canal, and natural hearing even in the portions of the spectrum for which there is little or no impairment must be foregone. Vent holes are sometimes provided to allow some normal sound field through but there is still substantial attenuation.
As a consequence, many persons with only high frequency hearing impairment find electronic hearing aids to be unsatisfactory, and simply accept the impairment as an unavoidable consequence of aging.
Some passive devices have been considered. Among these are devices constructed in the form or a Helmholtz type resonator cavity box with a small opening and a small exit that was then inserted into the outer ear and ear auditory canal. For various reasons, however, no commercial use has ever been made of such devices.
It is therefore clear that a need exists for an improved hearing enhancement device usable by those with high frequency hearing impairment for whom existing amplified devices are not completely satisfactory.